Molecular spectrometers are sometimes used to analyze the composition of materials. Such spectrometers emit light having known wavelength(s) and intensity onto the material to be analyzed, and then capturing the light scattered from (and/or transmitted through) the material, with the difference between the emitted and measured light providing information regarding the characteristics of the material. For example, near-infrared spectrometers direct different near-infrared wavelengths of light onto a material, either simultaneously or sequentially, and detect the intensity of the reflected or transmitted light at each wavelength. (Other wavelength ranges are possible, with ultraviolet, visible, or mid-infrared ranges—or some combination of ranges between 200 nm to 25,000 nm—being common.) The resulting spectrum returned by the material—that is, the intensity at each measured wavelength—can provide information regarding the composition of the material (or regarding other material characteristics, such as thickness, porosity, prior heat treatment, etc.) at the illuminated area. Further details can be found in, for example, Davies, A. M. C. et al: Near Infrared Spectroscopy: The Future Waves, NIR Publications (1996); and in Burns, D. A, et al: Handbook of Near-Infrared Analysis, Practical Spectroscopy Series, Marcel Dekker, Inc. (1992).
The spectrum returned by the illuminated area effectively represents the average material composition over the illuminated area; for example, if the illuminated area on a composite material has both fibers and polymeric material, the spectrum for this area will effectively be a combination of the spectrum for the fiber alone, plus the spectrum for the polymeric material alone. It is often more useful to know the spatial distribution of materials across a sample, rather than just measuring the “bulk” composition of the sample. Thus, a method known as spectral imaging—or hyperspectral imaging if a large number of wavelengths is analyzed—obtains spectra at sub-areas or “pixels” across the surface of the sample, where each pixel contains its own distinct spectral information. This (hyper)spectral imaging can be performed, for example, by uniformly illuminating a sample area with a sequence of wavelengths, and detecting the distribution of the composition at each pixel on the area using a camera sensitive in the wavelength range of the illumination.
(Hyper)spectral imaging devices require uniform and reproducible illumination across the sample area to be analyzed. Direct illumination using incandescent lamps is commonly used for cost-effectiveness, with quartz halogen lamps being popular owing to the wide wavelength range of quartz halogen sources. However, illumination from an incandescent lamp is nonuniform due to the structure of the lamp's filaments and bulb/enclosure, and due to any reflectors used to direct the light as needed. The use of multiple lamps introduces further nonuniformity in the distribution of light intensity and color due to the differences between the individual lamps, and due to the geometry of their relative placement.
Solid-state light sources such as LEDs and lasers are powerful light sources which produce a fraction of the heat of incandescent sources, and which typically have lifetimes extending to tens of thousands of hours. However, they are typically monochromatic sources—they emit in only a single wavelength, or in a narrow band—and typically a wider range of wavelengths is desired for spectral illumination. Thus, considerations for selecting and using solid-state sources are the availability and cost of the light sources capable of providing the desired wavelengths, and how to efficiently couple their light output to the sample area in a uniform manner. For example, a common method of seeking uniform output illumination from multiple input sources—whether solid-state or incandescent—is to couple the sources to fiber optics wherein the fibers are randomized from their input ends to their output ends. The large number of fibers and their random arrangement homogenizes the light, and provides a relatively uniform light spot at a distance from the output end. Such devices are commercially available from optical catalog companies, such as Edmund Optics (Barrington, N.J.). Each input end in these devices is usually a round fiber bundle, and the output end has fibers arranged in a round or linear pattern. This method works acceptably well for incandescent lamps, but for highly directed light sources such as lasers, the light coupling into the fiber is insufficiently uniform, and there is often a distinct laser speckle pattern on the sample being illuminated. Additionally, because the randomization of the fiber bundle is usually imperfect, there can be variations in intensity across the output side. A variation of this method uses a fiber bundle with one or more round input sides, and an output side wherein the fiber ends are arrayed in a ring-like configuration, with the fiber ends being oriented such that light output therefrom is oriented towards a common location. Here too illumination tends to be nonuniform at the output ends when highly directional light sources, such as LEDs and lasers, are used. Spatial uniformity of illumination is critical to the quality of spectral measurements, and each input wavelength needs to be projected on the area of interest with uniform intensity across the area.
Integrating spheres have previously been used to detect multi-directional reflection off of diffuse surfaces, as well as for measuring the absolute intensity of light-emitting devices. Companies such as Labsphere, Inc. (North Sutton, N.H.) produce differently-sized integrating spheres made from, or having their inner surfaces coated with, highly reflective materials. Input and output ports on the sphere provide input and output of light with a good degree of directional homogenization of the light due to the multiple reflections occurring inside the sphere. Integrating spheres have previously been used in near-infrared analyzers, such as the InfraAlyzer IA 450 (Bran and Luebbe, Norderstedt, Germany). Highly uniform illumination of the sample was achieved only if the sample was in contact with the output port of the integrating sphere; non-contact arrangements result in unsuitably nonuniform illumination. A small space between the illuminating device and the sample can be achieved by a half integrating sphere, such as the Hemilite™ Vision Illuminator (StockerYale, Inc., Salem, N.H.). This device has built-in LEDs and improves uniformity of illumination, but is not applicable to highly directional illumination, such as that provided by multiple lasers, because the light does not undergo the many multiple reflections that it experiences in complete integrating spheres.